The present invention relates to lamellar rare earth-iron-boron-based magnet alloy particles, a process for producing the rare earth-iron-boron-based magnetic alloy particles and a bonded magnet produced from such rare earth-iron-boron-based magnet alloy particles, and more particularly, to lamellar rare earth-iron-boron-based magnet alloy particles which have a residual magnetic flux density (Br) as high as not less than 10 kG, an intrinsic coercive force (iHc) as large as not less than 3.5 kOe and a maximum energy product ((BH)max) as large as not less than 13 MGOe, and which are excellent in rust preventability and leafing effect, a process for producing the lamellar rare earth-iron-boron-based magnet alloy particles, and a bonded magnet produced from such lamellar rare earth-iron-boron-based magnet alloy particles.
Bonded magnets which are advantageous in that they can be produced in any shape and have a high dimensional accuracy, etc., have conventionally been used in various fields such as electric appliances and automobile parts. With a recent development of miniaturized and light-weight electric appliances and automobile parts, bonded magnets used therefor have been strongly required to be miniaturized.
For this purpose, magnets have been strongly required to show a high magnet performance, i.e., a high residual magnetic flux density (Br), a large intrinsic coercive force (iHc) and a large maximum energy product ((BH)max).
As is well known in the arts, a bonded magnet comprising magneto plumbite type ferrite such as barium ferrite or strontium ferrite (referred to as xe2x80x98ferrite bonded magnetxe2x80x99 hereinunder) and a binder resin has an excellent rust preventability because ferrite particles are an oxide. In addition, since the ferrite bonded magnets are produced from a cheap material such as oxides of barium and strontium and iron oxide, the ferrite bonded magnets are economical and are, therefore, widely used.
As to the magnetic characteristics of these ferrite bonded magnets, however, the residual magnetic flux density (Br) is about 2 to 3 kG, the intrinsic coercive force (iHc) is about 2 to 3 kOe, and the maximum energy product ((BH)max) is about 1.6 to 2.3 MGOe. Therefore, these bonded magnets are insufficient to accomplish the miniaturization and weight-reduction of apparatuses or equipments in which the bonded magnets are incorporated.
On the other hand, there is no end to a demand for a higher performance and a lower price of a magnet. Since Nd-iron-boron-based magnet alloys using Nd which is relatively low in price among rare earth elements, have been almost simultaneously developed in 1982 by Sumitomo Tokushu Kinzoku Co., Ltd. (Japan) and General Motors Corp. (USA), the magnet alloys have been used in extensive application fields, and it has also been attempted to apply the magnet alloy to the production of bonded magnets. To further improve the magnetic characteristics, rare earth-iron-boron-based alloys for exchange-spring magnets have been earnestly developed and some of them have already been put to practical use.
An exchange-spring magnet exhibits a magnetic spring phenomenon by the exchange interaction of iron (xcex1Fe) or an iron compound and an Nd2Fe14B1 type tetragonal compound. Those magnets are characterized in a low rare earth element content and a high residual magnetic flux density (Br), and have a high possibility of being excellent on a cost/performance basis.
A rare earth-iron-boron-based alloy for exchange-spring magnets containing less than 10 atm % of a rare earth element such as Nd, has a high potential in magnetic characteristics as compared with a rare earth-iron-boron-based magnet alloy containing about 10 to 15 atm % of a rare earth element such as Nd which is in the vicinity of the stoichiometeric composition, e.g., commercially available xe2x80x9cMQPxe2x80x9d (trade name) developed by General Motors. Since it is possible to reduce the amount of expensive rare earth element used, this alloy is economically advantageous.
The rare earth-iron-boron-based alloy for exchange-spring magnets containing less than 10 atm % of a rare earth element such as Nd has a system containing xcex1Fe or a system containing Fe3B or Fe2B as the soft magnetic phase. The system containing xcex1Fe as the soft magnetic phase generally has a residual magnetic flux density (Br) as high as 10 to 13 kG, but the intrinsic coercive force (iHc) thereof is as low as less than 3.5 kOe at most. The system containing Fe3B or Fe2B as the soft magnetic phase generally has a comparatively high intrinsic coercive force (iHc) such as 3.5 to 7.7 kOe, but the residual magnetic flux density (Br) thereof is as low as less than 10 kG, and as a result, the bonded magnet produced from the system containing Fe3B or Fe2B as the soft magnetic phase has a higher residual magnetic flux density (Br) than that of xe2x80x9cMQPxe2x80x9d, but lower residual magnetic flux density (Br) than that composed of the system containing xcex1Fe as the soft magnetic phase.
In the field of small-sized motors for which bonded magnets produced from a rare earth-iron-boron-based magnet alloy is mainly used, bonded magnets are required to have well-balanced residual magnetic flux density (Br) and, intrinsic coercive force (iHc) from the point of view of miniaturization of motors and magnetic stability of the magnets used therefor. That is, bonded magnets are strongly required to have a residual magnetic flux density (Br) of not less than 10 kG and an intrinsic coercive force (iHc) of not less than 3.5 kOe.
On the other hand, a magnet alloy containing rare earth elements such as Nd is defective in that it is easily oxidized in the air and is likely to produce an oxide, so that the rust preventability is poor. Therefore, since bonded magnets produced from a magnet alloy containing a rare earth element such as Nd have a poor corrosion resistance, the bonded magnets are usually subjected to rust preventive coating-treatment such as dipping, spread coating or electro deposition using a resin and metal plating.
If the rust preventability of a magnet alloy containing a rare earth element such as Nd is enhanced, it may be possible to simplify or omit the rust preventive coating-treatment for the surfaces of bonded magnets even for the above-described use. In some uses, there is a possibility of omitting the rust preventive coating-treatment. Therefore, the enhancement of the rust preventability of a rare earth-iron-boron-based magnet alloy is strongly demanded.
The bonded magnets have also been produced usually by kneading magnet particles in a binder resin and forming the kneaded material into an appropriate shape. In this case, it is known that flake-like magnet particles are readily mechanically oriented, so that it is possible to enhance the packing density of these particles in the binder resin. However, in the case where the flake-like particles have curved surfaces, it becomes difficult to sufficiently enhance the packing density. In Japanese Patent Application Laid-open (KOKAI) No. 2-34706(1990), though the invention thereof relates to different application field from that of the present invention, it is described that xe2x80x9c . . . In general, as particles for paints, flake-like particles are preferred. That is, when such flake-like particles are mixed in a resin and the resultant paint is applied by a brush coating method or a spray coating method, these particles are deposited in parallel with the coating surface due to surface tension caused upon curing of the resin (called xe2x80x9cleafing effect or phenomenonxe2x80x9d), so that a continuous coating film composed of the particles is formed, thereby preventing the base material from coming into contact with outside air, and imparting a good corrosion resistance and weather resistance thereto . . . xe2x80x9d. Similarly, in the production of bonded magnets, when lamellar magnet alloy particles having no curved surfaces are used, the packing density of these particles in bonded magnets can be readily enhanced by the leafing effect thereof, whereby the residual magnetic flux density (Br) of the bonded magnet and as a result, the maximum energy product ((BH)max) thereof can be enhanced.
In consequence, it has been demanded to provide lamellar rare earth magnet alloy particles having no curved surfaces and exhibiting an excellent leafing effect.
More specifically, there has been a strong demand for lamellar rare earth-iron-boron-based magnet alloy particles which have a high residual magnetic flux density (Br), a large intrinsic coercive force (iHc) and a large maximum energy product ((BH)max), and are excellent in rust preventability and leafing effect.
In conventional quenched permanent magnet materials which contain Fe as the main ingredient (less than 91 atm %) and further contain at least one rare earth element (R) and boron (B), there is known a permanent magnet material which comprises less than 10 area % of a soft magnetic amorphous phase based on the total alloy structure and a crystalline phase as the balance which contains an Rxe2x80x94Fexe2x80x94B type hard magnetic compound (Japanese Patent Application Laid-Open (KOKAI) No. 8-162312 (1996)).
Although the production of rare earth-iron-boron-based magnet alloy particles which have a high residual magnetic flux density (Br), a large intrinsic coercive force (iHc) and a large maximum energy product ((BH)max), and are excellent in rust preventability and leafing effect, is now in the strongest demand, no rare earth-iron-boron-based magnet alloy particles having such properties are provided.
In the rare earth-iron-boron-based magnet alloy described in Japanese Patent Application Laid-Open (KOKAI) No. 8-162312 (1996)), the intrinsic coercive force (iHc) is as low as less than 3 kOe and the residual magnetic flux density (Br) is as low as less than 10 kG, as is clear from Table 5 in the specification in which the residual magnetic flux density (Br) is about 0.62 to 0.97 T (equivaleht to 6.2 to 9.7 kG), the intrinsic coercive force (iHc) is about 0.16 to 0.21 MA/m (equivalent to 1.25 to 2.6 kOe), and the maximum energy product ((BH)max) is about 19.7 to 72.0 kJ/m3 (equivalent to 2.5 to 9.0 MGOe).
The rare earth-iron-boron-based magnet alloys described in Examples 2 to 4 of Japanese Patent Application Laid-Open (KOKAI) No. 8-162312 (1996) are bulk bodies obtained by pulverizing a quenched ribbon and extruding the pulverized particles under a vacuum. The bulk bodies are, therefore, different in configuration from those of lamellar rare earth-iron-boron-based magnet alloy particles.
Accordingly, at present, it has been strongly demanded to provide lamellar rare earth-iron-boron-based magnet alloy particles which have a high residual magnetic flux density (Br), a comparatively large intrinsic coercive force (iHc) and a large maximum energy product ((BH)max), and are excellent in rust preventability and leafing effect for production of a bonded magnet.
As a result of the present inventors"" earnest studies, it has been found that by producing a mixture having a composition represented by the following formula:
RxFe(100xe2x88x92xxe2x88x92yxe2x88x92zxe2x88x92w)CoyMzBw
wherein R is at least one rare earth element selected from the group consisting of Nd, Pr, Dy, Tb and Ce, M is at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Mn, Cu, Ga, Ag and Si, x is 5 to 10, y is 1.0 to 9.0, z is 0.1 to 5, w is 2 to 7, (x+w): is not less than 9 and (y+z) is more than 5;
melting the obtained mixture under heating to,produce a molten alloy;
discharging the molten alloy through a nozzle;
spraying a gas onto the molten alloy discharged to form droplets of the molten alloy;
before solidification of the droplets thereof, causing the droplets to collide against a rotary cooling member disposed along the falling direction of the droplets to subject the droplets to quench solidification, thereby forming quenched and solidified particles; and
heat-treating the quenched and solidified particles in the temperature range of 600 to 850xc2x0 C.,
the obtained lamellar (including flat leaf-shaped and ellipse plate-shaped) rare earth-iron-boron-based magnet alloy particles have an average major axial diameter of 50 to 500 xcexcm, an average minor axial diameter of 50 to 500 xcexcm, an average axis ratio (major axial diameter/minor axial diameter) of 1 to 10 and an average aspect ratio (major axial diameter/thickness) of 5 to 100, exhibit a residual magnetic flux density (Br) as high as not less than 10 kG, an intrinsic coercive force (iHc) as large as not less than 3.5 kOe and a maximum energy product ((BH)max) as large as not less than 13 MGOe, are excellent in rust preventability and leafing effect, and are suitable for the production of bonded magnet.
The present invention has been attained on the basis of the finding.
It is an object of the present invention to provide lamellar rare earth-iron-boron-based magnet alloy particles for a bonded magnet, which have a high residual magnetic flux density (Br), a large intrinsic coercive force (iHc) and a large maximum energy product ((BH)max), and show an excellent rust preventability and an excellent leafing effect.
It is another object of the present invention to provide a process for producing lamellar rare earth-iron-boron-based magnet alloy particles with a high efficiency without a pulverizing step.
It is a further object of the present invention to provide a bonded magnet which has a high saturation magnetic flux density (Br) and a large maximum energy product ((BH)max), and shows an excellent rust preventability.
To accomplish the aims, in a first aspect of the present invention, there are provided lamellar rare earth-iron-boron-based magnet alloy particles for a bonded magnet,
having an intrinsic coercive force (iHc) of not less than 3.5 kOe, a residual magnetic flux density (Br) of not less than 9.5 kG, and a maximum energy product ((BH)max) of not less than 13 MGOe, and
having an average major axial diameter of 60 to 500 xcexcm, an average minor axial diameter of 50 to 460 xcexcm, an average axis ratio (major axial diameter/minor axial diameter) of 1.1 to 10 and an average aspect ratio (major axial diameter/thickness) of 3 to 100.
In a second aspect of the present invention, there are provided lamellar rare earth-iron-boron-based magnet alloy particles for a bonded magnet,
having an intrinsic coercive force (iHc) of not less than 3.5 kOe, a residual magnetic flux density (Br) of not less than 9.5 kG, and a maximum energy product ((BH)max) of not less than 13 MGOe,
having an average major axial diameter of 60 to 500 xcexcm, an average minor axial diameter of 50 to 460 xcexcm, an average axis ratio (major axial diameter/minor axial: diameter) of 1.1 to 10 and an average aspect ratio (major axial diameter/thickness) of 3 to 100, and having a composition represented by the formula:
RxFe(100xe2x88x92xxe2x88x92yxe2x88x92zxe2x88x92w)CoyMzBw
wherein R is at least one rare earth element selected from the group consisting of Nd, Pr, Dy, Tb and Ce, M is at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Mn, Cu, Ga, Ag and Si, x is 5 to 10, y is 1.0 to 9.0, z is 0.1 to 5, w is 2 to 7, (x+w) is not less than 9, and (y+z) is more than 5.
In a third aspect of the present invention, there is a process for producing lamellar rare earth-iron-boron-based magnet alloy particles for a bonded magnet, comprising the steps of:
preparing a mixture having a composition of the rare earth-iron-boron-based magnet alloy particles;
heat-melting said obtained mixture to produce a molten alloy;
discharging said molten alloy through a nozzle;
spraying a gas onto said molten alloy discharged to form droplets of said molten alloy;
before solidification of said droplets, causing said droplets to collide against a cone-shaped or disc-shaped rotary cooling member which is disposed along the falling direction of said droplets to subject said droplets to quench solidification and is rotated, thereby forming quenched and solidified particles; and
heat-treating said quenched and solidified particles in the temperature range of 600 to 850xc2x0 C.
In a fourth aspect of the present invention, there is provided a bonded magnet comprising:
85 to 99% by weight of the lamellar rare earth-iron-boron-based magnet alloy particles for a bonded magnet,
having an intrinsic coercive force (iHc) of not less than 3.5 kOe, a residual magnetic flux density (Br) of not less than 9.5 kG, and a maximum energy product ((BH)max) of not less than 13 MGOe, and
having an average major axial diameter of 60to 500 xcexcm, an average minor axial diameter of 50 to 460 xcexcm, an average axis ratio (major axial diameter/minor axial diameter) of 1.1 to 10 and an average aspect ratio (major axial diameter/thickness) of 3 to 100; and
a binder resin in which said lamellar rare earth-iron-boron-based magnet alloy particles are dispersed.
In a fifth aspect of the present invention, there is provided a bonded magnet comprising:
85 to 99% by weight of lamellar rare earth-iron-boron-based magnet alloy particles for a bonded magnet,
having an intrinsic coercive force (iHc) of not less than 3.5 kOe, a residual magnetic flux density (Br) of not less than 9.5 kG, and a maximum energy product ((BH)max) of not less than 13 MGOe,
having an average major axial diameter of 60 to 500 xcexcm, an average minor axial diameter of 50 to 460 xcexcm, an average axis ratio (major axial diameter/minor axial diameter) of 1.1 to 10 and an average aspect ratio (major axial diameter/thickness) of 3 to 100, and
having a composition represented by the formula:
RxFe(100xe2x88x92xxe2x88x92yxe2x88x92zxe2x88x92w)CoyMzBw
wherein R is at least one rare earth element selected from the group consisting of Nd, Pr, Dy, Tb and Ce, M is at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Mn, Cu, Ga, Ag and Si, x is 5 to 10, y is 1.0 to 9.0, z is 0.1 to 5, w is 2 to 7, (x+w) is not less than 9, and (y+z) is more than 5; and
a binder resin in which said lamellar rare earth-iron-boron-based magnet alloy particles are dispersed.